Described herein is a substantially colorless radiation curable overcoat composition that may be used for overcoating, for example, ink based images and xerographic based images. The overcoat composition described herein comprises at least one gellant, at least one monomer, at least one photoinitiator, optionally a curable wax, and optionally a surfactant, and is substantially free of colorants. Also described is a method for applying the overcoat composition to a substrate, for example a substrate that includes printed images thereon.
Wax based inks, sometimes known as phase change inks, are used to form digital images on paper using heated piezoelectric or acoustic ink jet heads. The ejection of an ink drop by the print heads is electronically controlled. In embodiments, the hot drop partially cools upon hitting an intermediate surface, often an aluminum drum. The complete image is assembled on the intermediate surface and then transferred to paper and fixed thereon with a combination of pressure and heat resulting in a solid ink, or wax-based ink print. Alternatively, the wax-based image can be printed directly to the substrate. The direct-to-paper image is also exposed to a combination of pressure and heat to fix the image to the substrate.
In conventional xerography, electrostatic latent images are formed on a xerographic surface by uniformly charging a charge retentive surface, such as a photoreceptor. The charged area is then selectively dissipated in a pattern of activating radiation corresponding to the original image. The latent charge pattern remaining on the surface corresponds to the area not exposed to radiation. Next, the latent charge pattern is visualized by passing the photoreceptor past one or more developer housings comprising toner, which adheres to the charge pattern by electrostatic attraction. The developed image is then fixed to the imaging surface or is transferred to a receiving substrate, such as paper, to which it is fixed by a suitable fusing technique, resulting in a xerographic print or toner-based print. To enable successful fusing with complete retention of the image on paper, in other words without offset of the image onto the fuser role, release enabling additives are incorporated in the process. Conventionally this release additive has been a silicone oil based fuser oil, more recently in some printer designs the complexity of handling fuser oil has been eliminated. This technological advance has been accomplished by incorporating wax into the toner particle. In both cases the fused image is left with a surface layer of either silicone oil or wax. Both surfaces can be difficult to subsequently coat.
Known methods of protecting toner-based images include applying an overcoat composition to the substrate. The overcoat composition is typically a liquid film coating that may be dried and/or cured. Curing may be accomplished through drying or heating or by applying ultraviolet light or low voltage electron beams to polymerize (crosslink) the components of the overcoat. However, known overcoat compositions fail to adequately protect toner-based prints and do not possess the requisite properties for controlled application, such as, for example, by an ink jet printer.
Typically, known overcoating formulations are applied using a liquid film coating device, and thus are often applied over the entire surface of the image, that is, flood coating. Applying a composition to part of an image, that is, spot coating, is possible, but it requires preparing a plate or cylinder prior to application of the overcoat composition. Thus, applying known coating formulations can be inefficient, difficult, and time-consuming and is not desirable for integration with variable-data digital printing.
Coating formulations for ink-based images are known. For example, UV curable ink-jet overcoats have been used in an attempt to overcome the failure of ink jet-generated images to withstand heat and sunlight. Typically, such UV curable ink-jet overcoats include polymerizable monomers, oligomers, or a mixture thereof. However, these UV curable ink-jet overcoats perform poorly when required to uniformly wet an ink-based image while simultaneously not increasing the transparency of the paper adjacent to the image.
A need exists for an ink jettable protective composition that provides overcoat coating properties including, for example, thermal and light stability, scratch resistance, and smear (or rub) resistance to toner-based images and ink-based images, particularly in commercial print applications. More specifically, a need exists for an overcoat coating that has a low viscosity (to enable ink jetting), yet is stable at the generally high temperature required for ink jetting and achieves the desired properties, including, but not limited to, the ability to wet over waxy surfaces from either solid ink jet prints or the waxy surfaces of toner fused in the absence of silicone oil, to remain on the surface of porous substrates and to not increase the transparency of the substrate, permit overwriting, reduce or prevent thermal cracking in response to thermal expansion of the toner, and protect an image from sun, heat, etc. In addition, a need still exists for a curable overcoating composition that does not yellow upon application and curing of the overcoating composition on a printed image.